The present invention relates to a time base error correction system for video signals reproduced from a rotary recording carrier such as a video disc.
In the reproduction of video signals recorded on a rotary recording carrier such as video disc, it is known that eccentricity of the rotary recording carrier or wow and flutter of the rotational mechanism may cause time base errors in the reproduced video signals resulting in interrefence in the reproduced video pictures. Particularly, when the video signals include color information, the acceptable limit for errors of frequency and phase in the sub-carrier is quite small and hence such time base errors must be reduced as much as possible.
A dominant frequency component of the time base error due to eccentricity of the rotary recording carrier is usually 30 Hz. One solution according to the prior art is to extract this 30 Hz component by means of bandpass filters from error signals obtained by phase comparison of the burst signal and to utilize the error signals in an independent closed-loop system so as to correct the time base errors. One disadvantage of this method is the design complexity of the bandpass filter which must have a very high Q (selectivity factor) so that the 30 Hz closed loop system will not interfere with the main control system. Another disadvantage is that if the time base error is larger, it takes quite a long time to lock in because of the time constant of the filter as well as the inherent nature of the closed-loop system. In the worst case, it may not lock at all. Further, in the case of CLV disc operated with a constant line velocity, since the dominant frequency component may shift downward with time from 30 Hz, the center frequency of the bandpass filter must be varied in accordance with the lapsed time. This gives rise to great difficulties in designing such a tracking filter and such a system may be unstable with respect to temperature changes. Furthermore, the conventional system can correct the time base errors to the extent of 15 microseconds at the most.
It is, therefore, the primary object of the present invention to provide a circuit arrangement for correcting time base errors in video signals which avoids the disadvantages of the prior art and extends the upper limit of the possible range of time base correction.